The invention relates to a particular construction of Savonius rotor blade, a particular Savonius vertical axis wind turbine rotor, and a drive for a driven element operatively connected to a Savonius rotor which automatically increases the effective gear ratio between the driving and driven elements in response to rotational speed of the driving element. The Savonius rotor blade, and rotor, according to the present invention have numerous advantages over prior art Savonius blades and rotors. In particular, because of the unique construction of the blades according to the present invention, a three bladed Savonius rotor is provided which can be expected to operate much more smoothly and effectively than conventional two bladed Savonius rotors, and be constructed in an overall better manner.
In the following specification and claims the following terms have the indicated meanings:                “Cp” or “maximum power coefficient” means (as it normally does): Turbine torque times turbine rotational speed divided by freestream dynamic pressure times freestream velocity times the turbine swept area; or proportional to maximum power divided by swept area [that is Cp=P/[1/2 A ρ v3] where P=power, A=swept area, ρ=the density of air (about 1.2 kg/m3 at sea level and 70 degrees F.), and v=wind velocity].        “Tip Speed Ratio” or “TSR” means (as it normally does): blade tip speed divided by wind speed. A drag rotor cannot have a TSR greater than one.        “Curvature” of a blade means: The ratio of the radius of the blade to the depth. The smaller the ratio, the more pronounced the curvature.        “Skew factor” of a blade means: The maximum curvature depth location along the radius of a blade. The larger the skew factor, the closer the maximum curvature depth is to the free end of the blade.        “Aspect ratio” means (as it normally does): The ratio of the length (height) of a rotor (or individual blade of a rotor) to its diameter.        “Effective gear ratio” means: The rpm ratio between a driving and a driven component, whether gears or some other mechanical structure (such as chains and sprockets, pulleys and belts, cones and belts, etc.) are used to provide the operative connection between the driving and driven components.        “Operatively” means (as it normally does): Any connection or engagement that allows the components connected or engaged to function as designed.        
Although from the time of filing his first patent application in 1924 (see canceled FIG. 6 of GB published specification 244,414) Sigurd Savonius—the inventor of the Savonius rotor—contemplated a three bladed version as well as two bladed versions, more than eighty years later there are few [e.g. see Environmental Building News, Vol. 13, #4, April, 2004, p. 7, “Solar and Wind-Powered Outdoor Lighting from MoonCell”] commercial versions of the three bladed version. Perhaps because extensive wind tunnel testing by Sandia Laboratories in 1977 [Blackwell et al, “Wind Tunnel Performance Data For Two And Three-Bucket Savonius Rotors”, SAND76-0131, July, 1977] concluded “The maximum power coefficient of the two-bucket configuration is approximately 1.5 times that for the three-bucket configuration” [Id. At p. 31], there has been almost no attempt to optimize a three bladed Savonius rotor. Conversely, there has been a great deal of work done on optimizing two bladed configurations [for example see Khan, “Model And Prototype Performance Characteristics Of Savonius Rotor Windmill”, Wind Engineering, Vol. 2, No. 2, 1978, pp. 75-85].
If a three bladed configuration of a Savonius rotor is optimized, the three bladed version can have advantages over and at least be competitive with two bladed versions. In addition to operating more smoothly, it can be just as easy or easier to manufacture; can have a Cp as great as, or greater than, two bladed versions with the same aspect ratio; and self-starts more easily. An important factor in the optimization of a three bladed Savonius rotor is the skew factor, something not even recognized as a result-effective variable for three bladed Savonius rotors in the prior art. It has been found that a high skew factor (e.g. at least about 0.6, preferably over about 0.7, and most preferably about 0.75-0.85), along with significant curvature, results in a rotor with a Cp about 2-5 times greater than those with similar curvatures but lower skew factors, e.g. 0.25 or 0.5 (about 0.5 being the common skew factor for three bladed Savonius rotors).
According to one aspect of the present invention there is provided a Savonius vertical axis wind turbine (“VAWT”) rotor comprising: A plurality of spokes, each spoke comprising a hub having a substantially central opening, three at least partially arcuate ribs extending substantially radially outwardly from the hub with inner and outer surfaces, and a plurality of channels defined in at least one of the inner and outer surface of each rib. A plurality of vanes of sheet material generally conforming to an inner or outer surface of a rib and having openings therein operatively aligned with the channels. And first fasteners passing through the openings into the channels and cooperating with second fasteners provided within the channels to securely hold the vanes to the ribs, so that the vanes assume an at least partially curved configuration presenting alternately a substantially concave and substantially convex curvature to wind as the rotor rotates about a substantially vertical axis.
The openings in the ribs are preferably non-tapped, and preferably the first fasteners comprise bolts and the second fasteners comprise nuts. Preferably, each spoke is in three pieces each piece comprising a hub segment and an arcuate generally radial rib. Two of the spoke pieces may be joined by a bridging piece, and two of the pieces may be joined by a clamping mechanism which draws the pieces toward each other to reduce the size of the central opening. Desirably a central shaft extends between the hub central openings, the clamping mechanism clamping the spoke hub to the central shaft. In one embodiment the clamping mechanism comprises a first fastener receiving element operatively connected to one of the spoke pieces at the hub segment, and a second fastener receiving element operatively connected to another, adjacent, spoke piece at the hub segment; and a fastener extending between the fastener receiving elements for drawing the elements toward each other to effect clamping.
More generally, each hub defines a clamp adapted to cooperate with a shaft so that the hub is securely affixed to the shaft. The clamp may be as described above, that is comprises surfaces of the hub defining a substantially radial slot in the hub communicating with the central opening; first and second fastener receiving elements on opposite sides of the slot and operatively connected to the hub; and a fastener extending between the fastener receiving elements to draw the surfaces of the hub together.
Preferably, the vanes generally conform to the outer surfaces of the ribs and are operatively connected thereto. Also, preferably each of the ribs has a free end opposite the hub, and a supporting element [e.g. strut or bar] extending between a central portion of the rib and a portion adjacent the free end thereof which increases the strength of the rib. Where three spoke pieces are provided, the rib of each spoke piece has a free end opposite the hub segment, and a supporting element extending between a central portion of the rib and a portion adjacent the free end thereof which increases the strength of the rib, and typically the spoke pieces are substantially identical.
The invention also relates to a substantially rigid spoke piece for a Savonius wind turbine comprising: a hub segment having an arcuate extend of roughly about 120 degrees and defining with two other spoke pieces a substantially circular opening; and a generally radial rib having a substantially convex surface and a substantially concave surface. The rib of the spoke piece has a free end opposite the hub segment, and preferably a supporting element extending between a central portion of the rib and a portion adjacent the free end thereof which increases the strength of the rib.
According to another aspect of the invention, a VAWT is provided comprising: A Savonius rotor comprising a plurality (preferably two or three) of blades having generally convex and concave surfaces operatively connected to each other, or a helical rotor. A driven element (such as an electrical generator or alternator, as disclosed in U.S. Pat. No. 6,172,429; a propeller, such as disclosed in co-pending application Ser. No. 10/443,954 filed May 23, 2003, a pump, etc.). And, a drive operatively connecting the driven element to the rotor; the drive automatically increasing the effective gear ratio as the speed of rotation of the rotor increases. [The maximum effective gear ratio is preferably at least about 10:1 when the driven element is a generator or alternator.] The Savonius rotor preferably further comprises at least one substantially vertical shaft operatively connected to the blades. Desirably, the drive directly senses rotor speed, or speed of an element operatively connected to the rotor, and does not and need not directly sense wind speed.
In one embodiment the drive comprises: A first sprocket operatively connected to the at least one shaft. Different size at least second and third sprockets, smaller than the first sprocket, and operatively connected to the driven element. A chain operatively connecting the first sprocket and one of the second or third sprockets. And a transmission comprising a centrifugal force responsive derailleur which automatically shifts the chain between the second and third sprockets. Especially where the driven element is a generator or alternator, the first sprocket and the third sprocket provide an effective gear ratio of at least 10:1, and the first sprocket and the second an effective gear ratio of less than 10:1.
While plural shaft versions of the Savonius rotor according to the invention—such as shown in co-pending application Ser. No. 10/854,280 filed May 27, 2004 (the disclosure of which is hereby incorporated by reference herein)—and other versions with spillover are within the scope of the invention, multiple shafts and significant spillover are not usually necessary when practicing the invention. That is, the Savonius rotor according to the invention may comprise a single shaft, with each spoke comprising a hub surrounding the shaft and operatively connected thereto to substantially preclude movement with respect to the shaft, the ribs extending generally radially outwardly from the hub.
It is a primary object of the present invention to provide an easily constructed and effective Savonius rotor having a wide variety of uses and used in a wide variety of manners while operating smoothly for effectively driving a number of different driven elements including a generator or alternator. This and other objects of the invention will become clear from a detailed description of the invention, and from the appended claims.